Various exhaust aftertreatment devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines. In the case of compression-ignition or diesel engines, a great deal of effort continues to be expended to develop practical and efficient devices and methods to reduce emissions of largely carbonaceous particulates otherwise present in the engine's exhaust gas.
An exhaust gas aftertreatment system for a modern diesel engine exhaust system typically incorporates a selective-catalytic-reduction device (SCR) and a Diesel Particulate Filter (DPF).
The SCR device is utilized to reduce NOx gases before the exhaust is discharged out a tailpipe. The SCR device uses a reductant capable of reacting with NOx gases to convert the NOx gases into inert byproducts, i.e., nitrogen and water. For example, the reductant can be an aqueous solution of urea, which is injected into the engine's exhaust stream. Once the reductant is in the exhaust stream, the reductant is absorbed into a catalyst of the SCR device, where the catalytic action of the SCR device ultimately converts NOx gases into the inert byproducts.
The DPF is utilized for collecting and disposing of the sooty particulate matter, emitted by the diesel engine, from the exhaust gas stream. A typical DPF acts as a trap for removing the particulate matter from the exhaust stream. The DPF may contain precious metals, such as platinum and/or palladium, which serve as catalysts to further oxidize soot and hydrocarbons present in the exhaust stream. In many instances, the DPF may be regenerated or cleaned using superheated exhaust gas to burn off the collected particulate.
Generally, the DPF is disposed downstream of the SCR device. In other words, the SCR device is spaced from the DPF such that the SCR device and the DPF are separate and independent components.